1. Field of the Invention
This invention relates to detonating cords with flash suppressing properties.
2. Summary of the Prior Art
Detonating cord is an item of commerce used to transmit a detonation front from one location to another, usually to initiate one or several charges of high explosives. Upon initiation of the cord, a highly exothermic explosion of the core occurs which travels from the point of initiation, the length of the cord, to initiate the explosive charge.
Standard detonating cords normally consist of a high explosive core of particulate pentaerythritol tetranitrate (PETN), occasionally cyclotrimethylenetrinitramine (RDX), trinitrotoluene (TNT), or pentolite (a mixture of TNT and PETN). The core is wrapped in yarns comprised of rayon, acetate or nylon fibers, and sometimes in tapes of polyester, ethylene vinyl acetate or other plastic material. The cord is then extrusion-coated with a plastic material such as polyethylene. Frequently the cord contains further exterior coatings of yarn and wax.
The inner yarns or tapes function to retain the particulate core material in place and are wrapped accordingly. The plastic coating surrounding the cord functions to waterproof the cord (a small amount of water will render PETN non-explosive), strengthen it and facilitate its ease of handling.
Generally, such standard cords are produced by either a "wet process" or a "dry process". In the former process, the explosive core is formed by a slurry of particulate PETN which is passed through a braiding machine to be encased in a braid, and then dried and subsequently coated; in the latter process, the explosive core is formed directly from dry particulate PETN which is encased in tape held closed by a yarn wrap; and subsequently coated as desired.
Where such standard detonating cord is laid through dry vegetation as is frequently the case in seismographic prospecting or clearing fields, the highly exothermic explosion (the "flash") of the core can and has initiated grass and forest fires.
Several approaches have been utilized in an attempt to solve this problem. One such approach includes providing a layer of inorganic salt such as chloride or phosphate salts around the braided or taped core. However, the use of such salts (plus sometimes water of hydration) results in a bulky and difficult-to-handle cord, the volume of salt required usually exceeding the bulk of the core. In addition, the entire length of cord must be carefully inspected to ensure that the coating is in place, a task complicated by the covering used to hold the salt in place.
A second approach is to extrusion-coat the braided or taped core with a halogenated polymer rather than polyethylene, to provide a solid, single-unit coating without breaks in it to suppress the flash. However, in order to extrusion-coat the core safely, temperatures below the melting point of the core (.about.286.degree. F. in the case of PETN) have to be used. As a result, highly halogenated polymers, such as polyvinyl chloride (PVC) cannot be effectively used; to be safely extrudable a PVC coating has to be diluted to such an extent that it is ineffective. Polymers with a lower degree of halogenation can be used, but the coating has to be relatively thick to be effective; this tends to add to the expense of preparing the cord and reduce its ease of handling. For example, using standard coating machines without modification, the cord generally must be coated twice with the halogenated polymer, in order for the coating to be effective with a core loading of greater than about 35 grains PETN per foot.
Other disadvantages of such halogenated extrusion coatings are that at high summer temperatures, for example, (about 100.degree. F. or more) the coatings frequently soften, deform, and subsequently crack; as a result, the cord may tend to absorb water at the cracks, or may even ignite nearby combustible materials. Also, the coatings have lower tensile strength than the formerly used polyethylene coating. Finally, these halogenated coatings frequently do not evenly and effectively absorb fluoroscent dyes often included as a safety measure to render the cord more visible.